This invention relates to an electronic circuit device provided with a ceramic substrate having lead pins and particularly relates to an electronic circuit device suitable for preventing breakage of the ceramics at the edge portion of the lead pin bonding pad.
In the prior art, silver solder (JIS (Japan Industrial Standards) Z 3261-1976) is used in the bonding of the input/output lead pins of a circuit in which an alumina ceramic substrate is used for securing high strength. With respect to this silver solder, approximately 10 types (melting point of approximately 600.degree. to 800.degree. C.) are known, namely B Ag-1 to B Ag-8A of JIS Z 3261-1976 as shown in Table 1.
TABLE 1 __________________________________________________________________________ Composition (wt %) JIS Total of symbol Ag Cu Zn Cd Ni Sn, Li other elements __________________________________________________________________________ B Ag-1 44-46 14-16 14-18 23--25 -- -- 0.15 or less B Ag-1A 49-51 14.5-16.5 14.5-18.5 17-19 -- -- Same as above B Ag-2 34-36 25-27 19-23 17-19 -- -- Same as above B Ag-3 49-51 14.5-16.5 13.5-17.5 15-17 2.5-3.5 -- Same as above B Ag-4 39-41 29-31 26-30 -- 1.5-2.5 -- Same as above B Ag-5 44-46 29-31 23-27 -- -- -- Same as above B Ag-6 49-51 33-35 14-18 -- -- -- Same as above B Ag-7 55-57 21-23 15-19 -- -- Sn 4.5-5.5 Same as above B Ag-8 71-73 27-29 -- -- -- -- Same as above B Ag-8A 71-73 Balance -- -- -- Li 0.15-0.3 Same as above __________________________________________________________________________
However, in the process of cooling the input/output lead pin after bonding thereof, from a temperature of 600.degree. to 800.degree. C. to room temperature, ceramics at the edge portion of the bonding pad break due to the thermal contraction stress of the solder. Even if the ceramics do not break, a large residual stress remains, thereby posing problems with reliability in service. One cause therefor is that the thermal stress cannot be adequately relaxed on account of the hardness of silver solder. As one method of solving this problem, there is proposed a bonding method by use of a Au-Sn-Ag system solder having a melting point of 280.degree. to 300.degree. C. (See, Japanese Patent Laid-Open 61-236148). However, this solder material is problematic in that it cannot satisfy the hierarchy of bonding temperatures required with Pb-Sn system materials, which are soft solders having melting points of 183.degree. C. to almost 320.degree. C. also used for making other bonding connections in the electronic circuit. In other words, the Au-Sn-Ag solder can not work without melting other solder materials during the assembly process.
Therefore, the prior art in general is problematic in that it can not meet the following three requirements simultaneously:
(1) The prevention of breakage of ceramics at the edge portion of the metallization pad on a ceramic substrate; PA1 (2) The reduction of residual stress in the solder; and PA1 (3) The maintenance of the hierarchy of bonding temperatures with Pb-Sn system solder.